Apparatus and method for producing silicon single crystals for semiconductor



March 5, 1.968 HIDEO YAMASE 3,372,003

I -APPARATUS AND METHOD FOR PRODUCING SILICON SINGLE CRYSTALS FOR SEMICONDUCTOR Filed July 17, 1964 2 Sheets-Sheet 1 Fig.2

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ATTORNEY United States Patent 3,372,003 APPARATUS AND METHOD FOR PRODUC- ING SILICON SINGLE CRYSTALS FOR SEMICONDUCTOR Hideo Yamase, Neda, Japan, assignor to Shin Nippon Chisso Hiryo Kabushiki Kaisha, Osaka, Japan, a corporation of Japan Filed July 17, 1964, Ser. No. 383,529 Claims priority, application Japan, July 19, 1963, $888,897 2 Claims. (Cl. 23301) ABSTRACT OF THE DISCLOSURE Method and apparatus for producing silicon single crystal by pulling a silicon single crystal from to 60% by weight of molten silicon in a crucible charged with polycrystalline silicon in one pulling cycle, transferring a pulling apparatus together with a pulled single crystal to a single crystal detaching position while simultaneously applying another pulling apparatus to the crucible and charging polycrystalline silicon in an amount corresponding to the pulled single crystal for use of subsequent pulling cycle and repeating the operation.

This invention relates to an apparatus and a method for producing silicon single crystal, by use of one crucible and a plurality of transferable pulling mechanisms.

In the production of silicon single crystal, there have been heretofore known following methods:

Pulling method (Czochralski method) Float zone method Epitaxial growth method It is well known that among the above mentioned methods the pulling method is excellent in the point of properties of crystal and low production cost, compared with other methods for producing silicon single crystals used in Zener diode, transistor and low voltage diode. In the pulling method, however, since quartz crucibles are used, heavy metals, various kinds of impurities, oxygen andt hel ike coming from the quartz crucibles generally exert following influences upon the crystals:

(1) Silicon single crystal with relatively higher resistivity is hard to obtain by this method, compared with the methods of float zone and others.

(II) As for the resistivity distribution on the transversal cross section of the pulled crystal, the ratio of the maximum resistivity (mainly produced in the vicinity of the periphery) to the minimum resistivity (mainly produced in the center) is comparatively larger than that produced by other methods.

(III) Moreover, the resistivity distribution in the axial direction of the crystal is determined by the segregation coefficient of the adulterant impurities in this silicon single crystal.

(IV) In this pulling method it is harder than other methods to produce silicon single crystal possessing narrow resistivity range except boron doped silicon single crystal.

Especially the fact that the resistivity ratios of the maximum to the minimum in both the traverse section and in the axial direction of the crystal has been causing poor yield in fabricating the pulled crystal, and considered to be one of the greatest obstacles in developing the application field of this crystal.

There is no doubt that continuous charge of polycrystalline crystals into crucibles is the most desirable method to improve this ratio in the axial direction from the theoretical viewpoint.

3,372,003 Patented Mar. 5, 1968 But on the other hand, it inevitably makes the charging as well as the construction of crucible complicated. In the pulling method, the expense for the crucible occupies relatively large part of the cost. The complicated mechanism and crucible brought about by the continuous charging mechanism of polycrystalline crystals further increases the expense for crucibles and prolonged operating time of pulling in one cycle leads to the risk of bringing more twins to the crystal.

On account of such technological and economical reasons, the continuous pulling method has never been used practically in commerce.

It is, accordingly an object of this invention to provide a method for producing silicon single crystal whose distribution of resistivity in the axial direction is comparable to that obtained by the ideal continuous charging pulling method and distribution of resistivity in the horizontal cross section is uniform, i.e. radial spread is narrow.

It is another object of this invention to provide a method for producing silicon single crystal possessing superior crystalline properties, which has no possibilities of forming twins even when operated in mass production manner.

It is a further object of the present invention to provide an apparatus which is simple in the mechanism but is capable to operate in mass production manner.

Other objects of this invention will appear hereinafter. The objects of this invention have been accomplished by repetition of the operation consisting of pulling a silicon single crystal from 10 to 60% by weight of molten silicon in a crucible charged with polycrystalline silicon in one pulling cycle, transferring a pulling apparatus together with a pulled up single crystal to a single crystal detaching position while simultaneously applying another pulling apparatus to the said crucible, and charging polycrystalline silicon in an amount corresponding to the pulled up amount of single crystal for use of subsequent pulling cycle. The method and the apparatus of this invention will be more fully understood referring to drawings.

FIG. 1 shows an elevation of vertical cross section taken along the line passing the center of turn table and the center of crucible of single crystal pulling apparatus according to the present invention.

FIG. 2 shows a plan of the sliding turn table, constituting the said apparatus.

FIG. 3 shows the theoretical resistivity distribution curves in axial direction of pulled silicon single crystals doped by boron, phosphorous or antimony.

FIG. 4 shows the actual resistivity distribution curves in axial direction of silicon single crystals doped by boron, phosphorous or antimony pulled by this invented method and apparatus.

FIG. 5 shows the relation between molten polycrystalline crystals in the crucible and a growing single crystal.

Polycrystalline silicon is charged from a charging hole 11 through a introducing pipe 12 to a quartz crucible 3 fixed in a carbon crucible 2, and molten therein. The introducing pipe 12 can be lifted up to the position not inconvenient to the pulling up operation during the time when this operation is being carried out while maintaining the air-tightness of the apparatus. Melting is carried out by use of a high frequenc induction heating coil 1 to which high frequency electric power is fed from a high frequency oscillator, not shown.

The temperature of the melt is detected by multiple thermo-couple radiation detector 5 connected to automatic temperature control system, not shown.

The quartz crucible has much larger capacity (the kg), and the crucible rotation system 4 makes the cru cible rotate so as to enable to heat by the high frequency heating coilun'iforrrily.

A piece of silicon single crystal seed 7 fixed to the tailor? the pulling shaft 6 as in the case of conventional type of apparatus, is dipped in the molten polycrystalline crystals. The rotation mechanism 8 of the pulling shaft and rotating mechanism 9 of pulling, work to pull up gradually the silicon single crystal, and the single crystal grows at the end of the seed in a bar form of specified shape and diameter. The growth can be controlled by the temperature controlling mechanism.

The cover 10 of the cone type furnace is made of transparent Pyrex glass or transparent quartz glass to make it easy to watch the single crystal being pulled.

- The upper part of the furnace is fixed to the board 14 supported by the leg 13.

At least two pulling mechanisms are fixed to the top surface of the fixing board, and the bottom surface of sliding turn table 16 is turned by the rotating shaft while sliding with-out leaking gas from the inside of the furnace by use of such elas-tomeric material as O-ring of Teflon rubber, Viton or the like as packing.

Each pulling mechanisms 18, 19 etc. fixed to the sliding turn table, at the position separated from each other at a suitable angle, has a set of mechanisms 6, 7, 8 and 9 respectively, and is capable to pull the silicon single crystal by itself.

Each pulling shaft is moved by turning the table from the place 18 to the place 20, from the place 19 to the place 18, after the shaft being pulled up at a position higher than the level of the sliding turn table.

At the place 18, the pulling shaft contacts the melted silicon and the pulling operation begins, but if it is required to cut off the crystal when it pulls 10 to 60% of the melted polycrystalline silicon in the crucible depending on its specification of the single crystal product required, by suddenly speeding up the pulling speed.

The shaft with the single crystal thus cut off is further pulled u wards above the level of the sliding turn table, and the sliding turn table is turned to move the pulling shaft from the place 18 to the place 21 and at the same time the pulling shaft at the place 19 where another pulling shaft having a piece of seed under its tail and wating its turn to pull the crystal comes to the place 18, contact the furnace and begins to pull the crystal.

These pulling operation is repeated continuously.

After pulling the single crystal, the polycrystalline silicon in the crucible may be left at molten state, but it is advisable to decrease the high frequency input and the temperature in the crucible to about 900 C. to solidify the molten polycrystalline silicon in the crucible in order that the subsequent operation can be carried out at stabilized condition.

The polycrystalline silicon are gradually supplied through 11 and 12 to the crucible, in an amount corresponding to the quantity pulled in the above-mentioned cycle.

After charging the polycrystalline silicon, the pipe 12 is slide back upwards to its position normally placed during the time when charging is not conducted so as not to prevent the pulling operation, and the high frequency input is increased gradually to melt the material in the crucible and to make the material ready to be pulled.

Then the pulling shaft is gradually lowered to dip the seed in the surface of the molten silicon, and the pulling operation is resumed again.

' When the pulling operation is finished, the pulling shaft is moved out of the furnace to the place of 20, where the pulled single crystal is taken off the shaft when the crystal is cooled ofii. And a new piece of seed is fixed to the tail of the shaft, to make it ready to resume the next pulling operation. FIG. 3 shows the tendency of the theoretical distribution of resistivity in the axial direction of the single crystal bar when doped phosphorous, boron or antimonyn The segregation coefficient (K) of each dopant is as follows:

P=O.35 (same as dopants As) Then the theoretical curves shown in FIG. 3 will be obtained,

p(O) means the top resistivity of the single crystal,

p(X) means the resistivity at the place of X% in the axial direction.

FIG. 4 shows the actual curves of resistivity, actually measured which is considered to be also influenced by the vaporized impurities from the molten polycrystalline silicon in the large crucible used in this invention. The curves in FIG. 4 is flatter than those in FIG. 3. The material which can be used for semiconductor is the part between or and {3 in FIG. 4 namely the fiat part of the resistivity.

A large crucible to which a large quantity of polycrystalline silicon can be supplied and a plurality of transferable puliing shafts have now made it possible to operate pulling of single crystals in an amount 10 to of all the molten polycrystalline silicon repeatedly by which the resistivity distribution from the top to the bottom of the pulled single crystal has become as flat as shown in the chart of FIG. 4 and accordingly the production of single crystal possessing narrow-ranged resistivity has been possible. These results are considered to be the same results as those obtained by the method of ideal continuous charging method. Moreover they make the appearance of star patterns in etch and pit caused by thermal distortion null, though the appearance of such phenomena cannot be avoided in pulling a single crystal from 100% by weight of the molten polycrystalline silicon by ordinary method namely, make it possible to obtain a single crystal of excellent crystallization properties.

Still more since the apparatus of the present invention has a plurality of pulling mechanisms, it is possible to take a emergency measure quickly.

When twins or other defects appears on the crystal during the time of pulling the crystal, it is possible to stop pulling the crystal and bring the defected crystal out of the furnace to replace it with another new shaft having a new seed under the tail to resume the pulling single crystal bar quickly.

Therefore the time loss and others caused by twins and other defects on the single crystal can be limited to the minimum, which is also a important factor which enable to use large crucible. Still more when a large crucible is used there is no more changing of the temperature required except the change of temperature setting on the shoulder of the crystal.

In FIG. 5, Do means a diameter of a crucible, Ds means a diameter of a growing single crystal, while 7 means a growing surface of the single crystal.

When a large crucible is used, Dc/Ds will be greater, and the distribution of temperature on the face of 'y will be even, so the radial spread of the resistivity will be much improved and naturally the device yield in the fabrication on the side of users will become much better. Though the explanation of this invention is referred to the production of the new ultra pure silicon single crystal, the same apparatus and the same method will be applied also to the mass production of single crystal of germanium or other metals by pulling method, at commercial scale.

In this explanation melting is performed by use of a high frequency induction heating furnace, but it is to be understood that a resistivity heating furnace can also be applied in very much the same way.

What is claimed is:

1. A method for producing ultra pure silicon single crystals from molten polycrystalline silicon crystals comprising a repetition of the steps consisting of pulling a silicon single crystal from 10 to 60 percent by weight of molten silicon in a crucible having capacity from one to three kg. charged with polycrystalline silicon in one pulling cycle, transferring a pulling apparatus together with a pulled single crystal to a detaching position when the above-mentioned pulling is finished while applying another pulling apparatus to the said crucible and charging polycrystalline silicon in an amount corresponding to the pulled quantity of single crystal for use of subsequent pulling cycle.

2. An apparatus for producing ultra pure silicon crystals from molten polycrystalline silicon crystals comprising a turn table which is rotatable around a central axis and a fixed table which supports a furnace and has elastomeric rings between the above-mentioned turn table for securing the tightness of the furnace, the above mentioned furnace having a crucible in the inside and equipped with a heating mechanism, a temperature measuring mechanism and a polycrystalline silicon crystals charging pipe and at least two pulling shafts which can be rotated and moved in a case communicated to the furnace when used in the pulling between the distance which enables its tail to touch the molten polycrystalline silicon in the crucible and to be lifted up to the level higher than the fixed bed forming the upper mouth of the furnace.

References Cited UNITED STATES PATENTS 2,753,280 7/1956 Moore 148--l.5 2,889,240 6/1959 Rosi 148--1.6 3,154,384 10/1964 Jones 1481.6

DAVID L. RECK, Primary Examiner. N. F. MARKVA, P. WEINSTEIN, Assistant Examiners. 

